1. Field of the Invention
The present invention relates to a method of manufacturing a microlens substrate, and a microlens exposure optical system. More specifically, the present invention relates to a method of manufacturing a microlens substrate for a projection-type liquid crystal display device that includes microlens arrays of a two-layer structure, and a microlens exposure optical system employed for this manufacturing method.
2. Description of the Related Art
A projection-type liquid crystal display device has excellent features, as compared with a projection-type CRT display device, of a wide color reproduction range, a small size and light weight and, therefore, handiness, no need of convergence adjustment because the device is not influenced by geomagnetism, and the like. In addition, since a screen of the device can be made larger easily, it is expected that the projection-type liquid crystal display device will be a leading home video display device.
As the projection-type liquid crystal display devices, those disclosed in Japanese Unexamined Patent Publication Nos. HEI 7 (1995)-181487 and HEI 9 (1997)-90336 to be described later in detail are known.
Examples of the projection-type liquid crystal display devices that display a color image using a liquid crystal display element include: a three-board liquid crystal display device using three liquid crystal display elements according to three primary colors of light; and a single-board liquid crystal display device using only one liquid crystal display element.
The three-board liquid crystal display device includes an optical system that divides white light into three primary colors of R, G and B, and the three liquid crystal display elements that control and combine respective color lights to form a color image, independently of each other. This liquid crystal display device optically superimposes the images of the respective colors and, thereby, provides full-color display.
With the configuration of this three-board liquid crystal display device, it is advantageously possible to effectively utilize light radiated from a white light source and ensure high color purity. However, this device has the following disadvantages. As described above, since the device requires a color separating system and a color combining system, the configuration of an optical system is complicated, the number of components increases, and it is difficult to realize cost reduction and size reduction.
On the other hand, the single-board liquid crystal display device is constituted to employ only one liquid crystal display element and to project the liquid crystal display element that includes filter patterns of three primary colors such as mosaic or stripe patterns. Since the number of liquid crystal display elements employed in the device is only one and the configuration of an optical system is simpler than that in the three-board liquid crystal display device, the single-board liquid crystal display device is appropriate for a low-cost, small-sized projection-type system.
However, the single-board liquid crystal display device has the following disadvantage. Since the color filters absorb or reflect colors, this device can use only about a third of incident light.
In order to solve these disadvantages, Japanese Unexamined Patent Publication No. HEI 7 (1995)-181487 discloses a color filter-less single-board liquid crystal display device that employs microlens arrays of a two-layer structure as shown in FIG. 11.
In this device, each of three dichroic mirrors 54G, 54R and 54B arranged in fan-shape divides white light emitted from a white light source 51 to respective colors of G, R and B, and makes the divided colors incident on two microlens arrays 4 and 7, arranged on a light source side of a liquid crystal display element 20, at different angles.
The light emitted from the white light source 51 is guided to the liquid crystal display element 20 via a curved mirror 52, a collimator lens 53, and the dichroic mirrors 54G, 54R and 54B.
Respective light fluxes that have passed through the first microlens array 4 are refracted by the second microlens array 7 so that main light beams of G, R and B divided by the dichroic mirrors 54G, 54R and 54B are almost in parallel to one another, and emitted to a liquid crystal region driven by a signal electrode to which corresponding color signals are applied independently, in a distributed manner.
The lights that have passed through the liquid crystal display element 20 are projected onto a screen 57 via a field lens 55 and a projection lens 56.
Since this device employs no absorption color filter, not only light utilization efficiency is improved but also the main light beams of the respective colors passing through the microlens arrays 4 and 7 are made almost in parallel. Accordingly, the spread of the main light beams of the respective colors until they reach the projection lens 56 is small, there is no a reduction in quantity of light caused by eclipse at the projection lens 56, and the device can provide an extremely bright image.
FIG. 12A is a plan view that illustrates an arrangement relationship among pixels, the first microlens 4, and the second microlens 7 in the simple-board liquid crystal display device shown in FIG. 11. FIG. 12B is a perspective view that illustrates a shape of a lens surface of the second microlens 7.
As shown in FIGS. 12A and 12B, the pixels are arranged at a constant pitch so as to correspond to a B component, an R component and a G component, respectively. The first microlens 4 and the second microlens 7 each correspond to a set of three pixels respectively corresponding to the B component, R component and G component.
The first microlens 4 is a spherical or aspheric axisymmetric lens having a hexagonal outline as indicated by a broken line in FIG. 12A. The second microlens 7 is a cylindrical lens having a rectangular outline as indicated by a solid line in FIG. 12A and having a convergence function in an X-axis direction. A black matrix 8, which is patterned as indicated by hatching, separates the respective color components of R, G and B.
Optical axes of the respective lenses that constitute the first microlens 4 and those of the respective lenses that constitute the second microlens 7 are in parallel to one another. The light axes of the opposite lenses of the first microlens 4 and the second microlens 7 are set coincident with each other.
The first microlens 4 converges incident light fluxes of three primary colors (R component, G component and B component), separated from one another with predetermined angle differences in advance, onto each set of corresponding three pixels. The second microlens 7 interposes between the corresponding first microlens 4 and the corresponding set of three pixels, and converts incident light fluxes inclined relative to the optical axis of the second microlens into incident light fluxes substantially parallel to the optical axis thereof.
Since the second microlens 7 is a cylindrical lens as shown in FIG. 12B, the second microlens 7 allows the first incident light flux (R component) that is originally parallel to the optical axis to move straight, collimates the second incident light flux (B component) inclined toward one side, and collimates the third incident light flux (G component) inclined toward the other side.
Microlens arrays having such a two-layer structure are formed by coupling them to both sides of one glass substrate, respectively. Alternatively, as disclosed in Japanese Unexamined Patent Publication No. HEI 9 (1997)-90336, the microlens arrays are formed by separately manufacturing the first microlens array and the second microlens array and coupling a lens formation surface of the first microlens array to a polished surface of the second microlens array.
However, the conventional microlens substrate having the two layers of microlens arrays is manufactured by coupling the respective microlens arrays to both sides of one glass substrate or by aligning two microlens substrates to each other as described above. The conventional microlens substrate has disadvantages in that it is difficult to make optical axes aligned to each other and production cost is thereby raised for the following reasons.
In the case of the microlens substrate having two layers of microlens arrays, it is necessary to execute a step of making optical axes of the two microlens arrays aligned to each other. In order to ensure optical characteristics of the lenses, it is required to make all of longitudinal directions, lateral directions and angles (rotating directions) of the two lens arrays aligned to one another. Nevertheless, because of fine lens patterns, an accuracy of about ±1 μm is required for the optical axis alignment, which makes it quite difficult to manufacture the microlens substrate of the two-layer structure.
The interposition of an intermediate layer between the two layers of microlens arrays also makes the optical axis alignment difficult. Namely, because of a gap between the two layers of lens patterns, it is impossible to observe both of positioning alignment marks of the two layers in focus simultaneously. It is possible to provide an individual alignment mark observation system for each layer and perform the optical axis alignment. In that case, however, it is also required to strictly make the optical axes of the alignment mark observation systems aligned to each other, which disadvantageously raises the cost of a positioning device.